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input output equations - 39.12

 

φ

M

GM

1

 

 

 

= -------

 

 

 

 

OC

-1

 

C

O

 

 

 

 

ω

c

 

 

 

 

A

 

 

 

 

-

 

 

 

39.9 NICHOLS CHART

- a plot of the gain vs. the angle of a system transfer function after a phasor trans-

form.

• The basic method is,

1.Write an expression for the closed loop gain Gc(s)G(s).

2.Apply a phasor transform to find the gain and phase angle.

3.Gain and phase values are calculated for a large range of frequencies and plotted on the following paper.

4.Measure the gain and phase margins from the plot.

input output equations - 39.13

 

 

 

 

 

 

 

 

 

 

 

0dB

25

 

 

 

 

 

 

 

0.5dB

 

 

 

 

 

-355°

 

 

 

 

 

 

-5°

 

 

 

 

 

 

 

 

 

 

 

20

 

 

 

 

 

 

 

 

 

 

 

15

 

-350°

 

 

 

 

 

 

 

-1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3dB

 

 

 

 

 

10

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-1dB

5

-330°

 

 

 

 

 

 

 

 

 

dB

 

 

 

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

-300°

 

 

 

 

 

 

 

 

 

-5

 

 

 

 

 

 

 

 

 

 

 

-10

 

 

 

 

 

 

 

 

 

-9dB

 

-270°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-90°

 

 

 

 

 

 

 

 

 

 

 

 

-15

 

 

 

-240°

 

 

 

 

-120°

 

 

 

 

 

 

-210°

-180°

-150°

 

 

 

 

 

 

 

 

-18dB

 

 

 

 

 

 

 

 

 

 

 

 

-20

 

 

 

 

 

 

 

 

 

 

 

-280

-260

-240

-220

-200

-180

-160

-140

-120

-100

-80

 

 

 

 

 

 

phase (deg)

 

 

 

 

 

 

 

• The gain and phase margins are measured from the plot as shown below,

 

input output equations - 39.14

180°

 

 

φ

M

 

 

 

0dB

GM

 

 

 

Where,

 

 

φ

M = phase margin

 

GM = gain margin

These correlate to values on the Bode plot. To find the approximate gain/phase margins with a Bode plot, locate the point where the gain is 180° to find the gain margin, and find the phase margin where the gain is 0dB.

In unstable systems the function will circle the center point of the plot.

39.10BESSEL POLYNOMIALS

-put in root locus chapter?

-The following table of functions provides polynomial roots normalized to a motion that is complete in 1second [How, 2001].

input output equations - 39.15

polynomial

roots

order

 

 

 

1

-4.6200

2

-4.0530±2.3400j

3

-5.0093,-3.9668±3.7845j

4

-4.0156±5.0723j,-5.5281±1.6553j

5

-6.4480,-4.1104±6.3142j,-5.9268±3.0813j

6

-4.2169±7.5300j,-6.2613±4.4018j,-7.1205±1.4540j

7

-8.0271,-4.3361±8.7519j, -6.5714±5.6786j,-7.6824±2.8081j

8

-4.4554±9.9715j,-6.8554±6.9278j,-8.1682±4.1057j,-8.7693±1.3616j

9

-9.6585,-4.5696±11.1838j,-7.1145±8.1557j,-8.5692±5.3655j,-9.4013±2.6655j

10

-4.6835±12.4033,-7.3609±9.3777j,-8.9898±6.6057j,-9.9657±3.9342j,-

 

10.4278±1.3071j

 

 

-the roots are used to construct the charateristic equation of the system.

-To increase, or decrease the reposnse time the root values are multiplied by the desired settling time.

39.11ITAE

-The Integral of Time Absolute Error (ITAE) integral equation becomes large for errors that persist for a long time.

JITAE = t e( t) dt

0

- is used to select poles to minimize the integral value when selecting a characteristics equation.